1. Field of the Invention
The present invention relates to a semiconductor memory device and, more particularly, to a memory device having an MIM nonlinear conductivity element.
2. Description of the Related Art
There is generally provided an MIM (metal-insulating film-metal) memory device in which a ferroelectric body having a theoretically infinite retention time of stored data is used as a recording/retaining section (storage medium) and a switch for controlling write/read access of the stored data is arranged. As the switch, a switch or an MIM switch is used. The MIM switch is a two-terminal switch for switching an ON/OFF state by using nonlinear current-voltage characteristics. In the nonlinear current-voltage characteristics, a current having a rapidly increased value flows when a voltage having a predetermined voltage value is applied.
In relation to the two-terminal switch, a ferroelectric memory using a MOS switch as a data input/output switch is described in "R. Womack et al Proc. of IEEE ISCC 1989 PP. 242-243".
As an insulating film used in the MIM switch, a Langmuir-Blodgett (LB) film obtained by stacking orientated monomolecular layers has been proposed.
For example, when 30 layers (120 .ANG.) made of a polyimide insulator are used as the LB film, the current-voltage characteristic curve of an MIM element, as shown in FIG. 16, is obtained. This current is constituted by a direct tunnel current or the like, a high-voltage region of the current-voltage characteristic curve corresponds to the Fowler-Nordheim current (to be referred to as an F-N current hereinafter) formula.
As shown in FIG. 6, in an energy diagram of electrons of the MIM element, assume that the thickness of the insulating layer is set to be "d" and that the height of an energy barrier is set to be ".phi.". At this time, when a potential difference V (.phi.&lt;eV) is applied across metal electrodes arranged on both the sides of the insulating layer, it is expected that a current value I of a current flowing into (passing through) the insulating layer depends on the following F-N current formula (1): ##EQU1##
Therefore, in an F-N conductive region, a relationship of log(I/V.sup.2) .infin.1/V is held, and the slope of the curve indicating the relationship is a function of ##EQU2## When the LB film is used, since the value d is estimated by the number of stacked monomolecular films, the value "m" can be calculated by the slope of an F-N plots.
That is, as shown in FIG. 17, it is found that the F-N plots have a linear portion in a high-voltage region. In this linear portion, the F-N current formula (1) is held, and the energy barrier height .phi.=0.65 eV is calculated by the slope of the linear portion.
A conventional MIM memory uses an insulating film (a direct tunnel layer or an FN tunnel layer) as a switching element to accumulate charges in a charge accumulating ferroelectric capacitor C. The insulating film allows a direct tunnel current to flow when a voltage having a predetermined value or more is applied to the insulating film. Thereafter, the insulating film having a recovered insulating property retains the charges by preventing leakage of the charges.
Therefore, a data retention time is determined by an insulating resistivity of the insulating film. when data must be retained for a long time, the insulating resistivity must be increased by increasing the thickness of the insulating film or using a resistor having a high resistivity.
However, a time required for increasing the thickness of the insulating film is increased since an insulating resistance layer is formed by the LangmuirBlodgett (LB) method. In relation to memory characteristics, a write voltage applied for allowing a tunnel current to flow is increased.
In addition, since write access is performed by not a direct tunneling phenomenon but an FN tunneling phenomenon, a write speed is decreased by a decrease in charge injection efficiency.
Therefore, characteristics such as a long data retention time, a low driving voltage, and a short processing time cannot be easily obtained in a conventional memory structure.
In the F-N current, all electrons for applying a voltage V in a region of .phi.&lt;eV do not flow through (tunnel-conducting) the insulating film in the direction of thickness, and after they flow through a part in this direction, they flow at a conductive level of the insulating layer. The insulating film is damaged by heat generated when the electrons flow through the insulating layer. A decrease in breakdown voltage, and the like occur.
Therefore, in order to obtain satisfactory nonlinear characteristics, the value "eV" must be much larger than the value ".phi.". That is, an operation voltage must be increased.
However, when memory elements are to be integrated, the high operation voltage is a disadvantageous factor to obtain an increase in integration density of the elements, an increase in an operation speed, stability of repetitive operations, and a decrease in power consumption.